They're a nightmare, aren't they? I cut the sticky seal off the bottom of the packaging so it doesn't stick to the letters on their way back in. That helps.

Also some are worse for falling off the packaging than others. The bad ones, I cut pieces of clear acetate to fit in the package, and hold those over the back and front as I slide them into the package. I store them upright, upside-down and closely packed together to help stop the letters falling off.